Cassandra says "ClusterName mismatch: oldClusterName != newClusterName" and refuses to start
To prevent operator errors, Cassandra stores the name of the cluster in its system table. If you need to rename a cluster for some reason, you can:
Perform these steps on each node:
Start the cassandra-cli connected locally to this node.
Run the following:
use system;
set LocationInfo[utf8('L')][utf8('ClusterName')]=utf8('');
exit;
Run nodetool flush on this node.
Update the cassandra.yaml file for the cluster_name as the same as 2b).
Restart the node.
Once all nodes have been had this operation performed and restarted, nodetool ring should show all nodes as UP.